Semiconductor processing typically includes several Chemical Mechanical Polishing (CMP) steps. CMP combines the chemical removal effect of an acidic or basic fluid solution with the “mechanical” effect provided by polishing with an abrasive material. The CMP system usually has a polishing “head” that presses the rotating wafer against a flexible pad. A wet chemical slurry containing a micro-abrasive is placed between the wafer and pad.
CMP removes material from uneven topography on a wafer surface until a flat (planarized) surface is created. This allows subsequent photolithography to take place with greater accuracy, and enables film layers to be built up with minimal height variations.
A new CMP application has been introduced recently where CMP is used to clean up fencing and resist remaining from prior processing steps. For example, in disk head fabrication, a CMP liftoff process with SiO2 slurry and standard hard polishing pad is implemented for sensor track width definition. However, this process is reaching the end of its process capability as sensor track width continues to shrink. Current CMP processes cannot completely remove fencing and/or resist in the critical track width due to the topography formed by resist becoming thinner and narrower. Current process of record (POR) CMP liftoff process have been found to not effectively remove lead shorting and fencing, and cause lead resistance variation and sensor instability for narrow track products.
The known polishing pads for the mirror surface of a semiconductor wafer used in CMP include a polishing pad of polyurethane foam type, a polishing pad of polishing cloth type having a polyester nonwoven fabric impregnated with polyurethane resin, and a polishing pad of stacked type having the above two pads laminated therein.
For the polishing pad of polyurethane foam type, a polyurethane foam sheet having a void volume of about 30 to about 35% is typically used. A polishing pad comprising fine hollow particles or water-soluble polymer particles dispersed in a matrix resin such as polyurethane are also known.
Among these polishing pads are those formed with grooves or holes on the surface of their polishing layer for the purpose of improving the fluidity of slurry and maintaining the slurry.
The known polyurethane foam sheet having a void volume of about 30 to about 35% as described above is excellent in a local planarization, but exhibits low compressibility, i.e., on the order of about 0.5 to about 1.0% and is thus poor in cushioning characteristics, making it difficult to exert uniform pressure onto the whole surface of a wafer. Accordingly, CMP processing is carried out usually after the backside of a polyurethane foam sheet is provided separately with a soft cushion layer.
However, none of the above-mentioned polishing pads have been able to provide satisfactory removal of resist and fencing from the high aspect ratio channel formed between sensor leads adjacent the sensor track width. The following discussion describes the problem in more detail.
FIG. 1 is a top down view of a wafer 100 of magnetoresistance (MR) sensors. The track width (W) of the sensor is defined between the leads 102, 104. CMP dislodges particles 106 of resist and fencing. These particles 106 can lodge in between the leads 102, 104, causing a short. However, CMP with standard hard pads cannot always remove particles from between the leads 102, 104. If the particle cannot be removed, the short causes the device to fail. Additionally, as shown in FIG. 2, traditional CMP tends to remove the edges of the leads, causing a reduction in the overall cross sectional area of the lead. This in turn causes an increase in the resistance of the lead, and resultant loss of sensitivity of the MR sensor. Further, as shown in the microscopy scan of a representative MR sensor (FIG. 3), which corresponds generally to the cross sectional view shown in FIG. 2, traditional CMP cannot completely remove fencing 300. A fence next to the sensor affects sensor performance by creating an electrical short between the sensor and the shield and also by changing the gap and shield coverage profile above the sensor.
What is needed is a way to perform CMP which reduces or avoids these adverse effects.